This invention relates to transport mechanisms for moving a print head relative to a print receiving medium.
Print head transport mechanisms are known in which a digital print head is traversed alternately in opposite directions across the width of a print receiving medium. Such mechanisms are used in computer output printers in which a plurality of lines of printing are effected on a sheet of paper. After a traverse of the print head across the print receiving medium in which a line of printing is effected, the print receiving medium is fed, in a direction perpendicular to the traverse of the print head, so as to move the line of printing away from alignment with the line of traverse of the print head and to bring an unprinted portion of the print receiving medium into alignment with the traverse of the print head. The print heads may be impact wire dot print heads, ink jet print heads, thermal print heads or other types of print head in which a plurality of print elements are selectively operated to effect printing. The wires of the impact wire dot print heads are aligned in a row and likewise the ink jet print heads include a plurality of ink jet nozzles aligned in a row and thermal print heads include a plurality of thermal printing elements aligned in a row. Selective actuation of the wires, nozzles or thermal printing elements enables printing of dots of ink at selected positions aligned with the row of print elements. The traverse of the print head is perpendicular to the row of print elements and hence, as is well known, selective actuation of the print elements during traverse of the print head is effective to print desired characters and patterns on the print receiving medium. It will be appreciated that the maximum height of character that can be printed in a traverse of the print head is determined by the length of the row of print elements.
It is proposed to use ink jet print heads for printing postal indicia on mail items. However the required height of the postal indicia is greater than the length of the row of ink jet nozzles in commonly commercially available ink jet print heads.
According to the invention a print head transport mechanism includes a carriage mounted on a chassis for traversal relative to said chassis in a first direction and in a second direction opposite to said first direction over and beyond a print receiving area; means for driving the carriage alternately in said first and second directions; a sub-carriage for mounting a print head, said sub-carriage being mounted on said carriage for traversal relative to the carriage in a third direction substantially parallel to said print receiving area and transverse to said first and second directions; a guide mounted on the chassis; means on the sub-carriage engaging with said guide; said guide having a formation such as to maintain the sub-carriage in a first index position when the carriage is traversed in the first direction, to maintain the sub-carriage in a second index position displaced from the first index position when the carriage is traversed in the second direction, to effect traverse of the sub-carriage from said first index position to said second index position each time the carriage is traversed in said first direction beyond the print receiving area and to effect traverse of the sub-carriage from said second index position to said first index position each time the carriage is traversed in said second direction beyond the print receiving area.